1. Field of the Invention
The present invention relates to a multi-headed cryopump apparatus in which plural cryopumps are driven by a common compressor.
2. Description of the Prior Art
A multi-headed cryopump apparatus of the conventional type is disclosed, for example, in Japanese Laid-Open Patent Application No. 63-57881. In this conventional apparatus, an encoder for detecting the operating position of each cryompump, which is driven according to Gifford-McMahon cycle, is employed for assuring equal supply of the operating fluid effectively to each cryopump. This is accomplished even through each opening of the valve of each cryopump brings a temporary decrease in compression-ratio, which has an effect on the entire system. The position of the motor which controls a cam-operated valve or the condition of the valve itself is detected by the encoder for controlling the current to the motor by a control unit. The control unit responds to the signals from the encoder so that while one of the cryopumps is in its in-take stroke for intaking operating fluid under a high pressure, no other cryopump is in its in-take stroke.
However, in the conventional apparatus, the encoder has to be equipped in each cryopump, thereby requiring considerable modification of each cryopump at a high cost. In addition, such modification requires that cables be interposed between each cryopump and the control unit, whereby it is difficult to establish a remote-control system for the whole apparatus.